The present invention relates in general to a dot-matrix print head, and more particularly to a structure for pivotal movements of armatures for longitudinal or endwise movements of print wires.
In a known dot-matrix print head, each of a plurality of armatures supporting respective print wires is pivotally supported for endwise movements of the print wires, upon energization and deenergization of respective electromagnets which are accommodated in a cylindrical yoke in circumferentially spaced-apart relation with each other. The armatures are disposed radially such that the distal end of each armature to which the print wire is secured is disposed in the central part of the cylindrical yoke. In such type of dot-matrix print head, a structure for pivotal support of each armature includes a support pin erected on or near the cylindrical wall of the yoke. Each armature has a hole in which the corresponding pin is loosely fitted, so that the armature is pivotable about the pin or hole, in a plane parallel to the pin (in a plane perpendicular to the plane of the armature), when the corresponding electromagnet is energized to magnetically attract the armature. In this arrangement, the print wires connected to the distal ends of the armatures are selectively activated through selective energization of the corresponding electromagnets according to printing data, whereby desired images such as characters may be printed in a matrix of dots formed by the free ends of the activated print wires.
The print wires are returned to their non-operated positions upon deenergization of the corresponding electromagnets, with biasing forces of biasing springs acting on the corresponding armatures. The print wires are held in their non-operated positions by the biasing springs.
In the case where the support pins for the armatures are fixed in holes formed in the cylindrical wall of the yoke, the wall should be formed with a large enough thickness to form the holes for the support pins. This results in an increase in the size and weight of the print head. In the case where the support pins are fixed in holes radially inwardly away from the cylindrical wall of the yoke, there exists a large distance between the support pins and the cylindrical wall of the yoke at which the armatures are pivotally supported. The distance between a hole to be formed in each armature for the support pin and the proximal end face of the armature must be large enough to maintain a sufficient strength of the armature at its proximal end portion during formation or cutting of the hole in order to permit the hole to be properly formed. Otherwise, the proximal end portion of the armature may be deformed due to a force exerted during the formation of the hole. Further, the support pin must have a sufficient diameter to provide a sufficient mechanical strength, and consequently the pin hole in the armature must have a correspondingly large diameter. For the reasons stated above, the pin-and-hole arrangement for the armature requires a relatively large distance between the support pin (point of contact of the armature with the pin) and the cylindrical wall of the yoke at which the armature is pivotally supported. Therefore, the pivotal movements of the armature about its proximal end will cause a relatively large amount of relative movements between the armature and the support pin engaging the pin hole, whereby the armature and the support pin are subject to large amounts of friction and wear. A frictional resistance to the movements of the armature relative to the support pin will cause a variation in the operating force of the armature. Further, the wear of the armature and the support pin will cause a variation in the position of the distal end of the armature at which the print wire is fixed. Accordingly, the operating stroke of the print wire is varied, resulting in a variation of the printing force of the print wire against a recording medium. Thus, the dot-matrix print head using the pin-and-hole support structure for the armatures suffers from inconsistent or unstable quality of printing by the print wires, due to a large distance of the support pin from the proximal end of the armature at which the armature is pivotally supported.
Another factor for inconsistency of the printing forces of the print wires lies in that the print wires are held in their non-operated positions with different values of forces acting on the armatures, even though the biasing springs for returning the print wires to their non-operated positions have the same free length and the same spring constant. Namely, a force which must be overcome by a magnetic force produced by an electromagnet for one armature is different from that which must be overcome by a magnetic force produced by an electromagnet for another armature. This difference is caused by a difference in the amount of deflection of the print wires from one wire to another. More specifically, the fixed ends of the print wires secured to the distal ends of the armatures are arranged along a circle, while the free ends of the armatures are generally arranged in one or two straight rows. Therefore, the print wires are elastically deformed in different degrees between their fixed and free ends. While the biasing springs primarily serve to hold the armatures in their non-operated positions, the print wires function more or less to apply biasing forces to the armatures, due to their elastic deformation. The biasing forces of the print wires which are produced due to their elastic deformation are different from each other because the amounts of deflection of the print wires are different from each other. For this reason, the armatures receive different total values of biasing forces.